Retrieve value by variable name in erlang function

Question

Is it possible somehow to retrieve variable value by its name (name represented as string)?

% we are calling foo function as foo(3)
foo(Param) ->
    Var1 = Param * 2,
    % some magic code here which can evaluate 
    % "Var1" string to Var1's value (6)
    ok.

I want to implement (if it is possible) some kind of logger macro, like

Param = 3*4,
% This should write "My parameter value is 12" to log
?LOG("My parameter value is $Param").

Thanks.

If you are very specific about about logging you can use existing library called lager. It is widely used logging library which uses parse_transform. — Marutha
– Marutha, Commented Dec 21, 2013 at 7:33
@Marutha I am not very specific about logging. I just want to have convenient way to log message (without writing a lot of tildes and brackets in code: io:format("My parameter value is ~p~n", [Param]).) . — sergeyz
– sergeyz, Commented Dec 22, 2013 at 19:42

Dmitry Belyaev · Accepted Answer · 2013-12-20 22:56:10Z

1

The common way to log is to have formatting string and list of parameters. However your idea is achievable through usage of parse transform.

answered Dec 20, 2013 at 22:56

Dmitry Belyaev

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sergeyz Over a year ago

Thanks! Parse transform is what I need.

sergeyz · Accepted Answer · 2013-12-22 20:01:14Z

1

Thanks to Dmitry Belyaev for mentioning parse transform. Say we have logging code:

?dump("My parameter value is $Param")

What I need here is to parse variables within format string ("My parameter value is $Param") with some regular expression. This format string contains single var name (Param). And we need to insert io_lib:format function call (by transforming original AST) with modified format string:

print_message(io_lib:format("My parameter value is ~p~n", [Param]))

In result we can archive required behavior:

Bar = "hello",
Buzz = buzz123,    
?dump("Say $Bar to $Buzz"),
% => example:19: Say "hello" to buzz123

You can look at my implementation here

answered Dec 22, 2013 at 20:01

sergeyz

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Comments

Tilman · Accepted Answer · 2013-12-20 18:18:36Z

0

For toy problems you could use:

io:format("My parameter value is ~p~n", [Param]).

See io_lib and io.

Alternatively:

error_logger:info_report/1

or other error_logger functions.

The logging library lager is commonly used.

edited Dec 20, 2013 at 18:18

answered Dec 20, 2013 at 18:08

Tilman

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2 Comments

sergeyz Over a year ago

Code like io:format("My parameter value is ~p~n", [Param]). is really annoying sometimes (all those tildes and brackets...) Thats why I would like to use simpler code like ?LOG("My parameter value is $Param")

Tilman Over a year ago

I see, though all your code will depend on your parse transform. Parse transforms can be hard to get correct, check out parse_trans (github.com/uwiger/parse_trans). io_lib with all those ~s helps with formatting, it separates the format from the data. The syntax you propose is "unusual" for Erlang code, but certainly readable.

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